1. Field of the Invention
The present invention relates to a shift control apparatus for an automatic transmission.
2. Description of the Related Art
In conventional automatic transmissions, a power transmission route in a shift device including a planetary gear unit is switched by means of engagement/release of engagement elements, in order to change a gear ratio for producing a plurality of shift speeds. When upshifting or downshifting in order to realize a specific shift speed, for the purpose of simplification of the engagement/release of the engagement elements and inhibiting of shift shock, another engagement element is additionally engaged, or a predetermined engagement element under engagement is released, with respect to the existing engagement of a plurality of engagement elements or a single engagement element. Due to the structure of the gear train constituting the shift device, when unavoidable, a change over operation for an engagement element that engages another engagement elements is executed, while an engagement element under engagement is released.
Recently, there has been a tendency to increase the number of speeds of the automatic transmission in order to improve drivability and enhance fuel economy. For this purpose, an over-drive gear or an under-drive gear is typically incorporated in a shift device including a plurality of planetary gear units. By doing so, a shift speed serving as an acceleration or a deceleration speed is added.
Further, an automatic transmission achieving multiple speeds by means of splitting an input to a Ravigneaux type planetary gear set into a high-low dual system has been disclosed (see Laid-open Japanese Patent Application No. Hei.4-219553).
However, such conventional automatic transmissions require a complicated multiple change over operation for four engagement elements instead of the simple change over operation for two engagement elements, because of the wide selection of shift speeds to match a running condition of a vehicle. Examples of the need to execute the multiple change over operation for the four engagement elements include shifting from a predetermined shift speed to a specific shift speed without involving the shift speed adjacent thereto, namely, a “jump shift”.
In the multiple change over operation, a way of controlling the order, timing and the like, in which the individual engagement elements are engaged and/or released is of extreme importance. If the engagement elements are not engaged/released in the correct order and with the proper timing, and the like, a smooth shift cannot be achieved, and therefore shift continuousness is impaired. As a result, problems result such as the occurrence of step-like shocks during shifting, with a particularly substantial shock at the completion of shifting, or alternatively, the time required for shifting becomes longer than desired.